This invention relates to an ultrathin permselective polyetherimide membrane and a gas separation process employing the membrane.
Methods for preparing very thin polymeric films are described in U.S. Pat. No. 3,580,841 (Cadotte, et al.) at column 5, line 54 to column 6, line 29, U.S. Pat. No. 3,767,737 (Lundstrom), Canadian J. Chem. 33, 15 (1955) (Pate and Yaffee article entitled "A New Material and Techniques for Fabrication and Measurement for Very Thin Films for Use in 4.pi.-Counting"), and in U.S. Patent Application Ser. No. 536,650 (Ward), filed Dec. 26, 1974, assigned to the assignee hereof and incorporated herein by reference.
Polyetherimides C.) by reacting certain aromatic bis(ether anhydrides) and organic diamines are described in U.S. Pat. No. 3,847,867 (Heath and Wirth). A high-temperature method (greater than 130.degree. C.) for making such polyetherimides is described in U.S. Pat. No. 3,991,004 (Takekoshi et al.), which issued on a continuation-in-part of Application Ser. No. 319,371, filed Dec. 22, 1972. As described generally in the Heath et al. patent, the polyetherimides disclosed therein, when made by the high-temperature method disclosed in the last-mentioned application, can be cast from an organic solvent solution to a thermoplastic film. In Example 5 of Heath et al., preparation of a flexible film is described by doctor blading onto a glass slide to a thickness of 1 mil a mixture of 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl] propane dianhydride (4,4'-BPADA), 4,4-diamino-dipenyl ether (ODA), and dimethylacetamide (after stirring at ambient temperature for three hours) and thereafter evaporating the solvent. Example 6 thereof sets forth a similar preparation of a related film using 2,2-bis[4-(2,3-dicarboxyphenoxy)phenyl] propane dianhydride (3,3'-BPADA), ODA, and dimethylformamide as solvent.
However, the prior art does not disclose ultrathin (i.e., less than about 10 microns in thickness) permselective membranes formed from polyetherimides prepared from BPADA and meta-phenylene diamine (m-PD).
It has now been found by practice of the present invention that such ultrathin permselective polyetherimide membranes can be formed in simple efficient manner. The resulting membranes are found to have high permeabilities to a variety of gases. These membranes may be usefully and effectively employed to effect selective removal of such gases from mixtures thereof with other gases.